The present invention relates to a fuel injection system for an internal combustion engine, and more particularly to a multi-injection system.
In the multi-injection system, a fuel injection valve is provided adjacent to an intake valve of each cylinder. In a conventional four-cycle four-cylinder engine, fuel is injected from all injection valves at every other ignition time, in other words fuel is injected once per revolution of the crankshaft of the engine at the ignition time for one of the cylinders.
FIG. 4 shows a time chart of fuel injection timing of a conventional multi-injection system for an engine (but not the curves labeled FUEL INJECTION I and II).
In the system fuel is injected at ignition time a for a number 1 cylinder and at ignition time b for a number 2 cylinder. Considering the fuel injection for the numbers 1 and 3 cylinders, the fuel injection at the ignition time a takes place immediately before the end of the intake stroke of the number 3 cylinder. It is also the same for the number 2 and 4 cylinders.
Generally, a part of the air fuel mixture induced into the cylinder during the intake stroke flows back to the common intake manifold at the end of the stroke. Therefore, a part of the fuel injected into the number 3 cylinder at the ignition time a is carried back by the returning air to the intake valve of the number 1 cylinder through the intake manifold and added to the fuel injected from the injection valve for the number 1 cylinder. Therefore, a comparatively large amount of fuel is stored in the space above the intake valve of the number 1 cylinder and that fuel is induced into the number 1 cylinder during the intake stroke of the cylinder. Accordingly, fuel is unequally distributed to the cylinders.
A fuel injection system has been proposed which operates to delay fuel injection at a time after the induced air has flown back so as to uniformly distribute the fuel to all cylinders of an engine. However, the amount of returning fuel decreases with an increase in engine speed. Further, it is difficult to control the delay time at high engine speed, since the period between fuel injection times becomes shorter.